Formation of a patch of altocumulus with irregular gaps in this cloud cover due to an upper-level trough over the NW Mediterranean moving rapidly southeastwards and therefore introducing colder air over the region hence causing the warmer air to slide over the cooler air at mid-levels generating instability hence wool of cotton-like altocumulus. The first thumbnail shows another section of this cloud seemingly producing light precipitation (virga) at the cloud`s edge in the background. The second and third thumbnail shows the rapid introduction of cold air at 500mb height.